The evolution of vibrational phase space during the collisional relaxation of highly excited collinear CS2

Comparison of intermolecular energy transfer from vibrationally excited benzene in mixed nitrogen-benzene baths at 140 K and 300 K

Gas phase intermolecular energy transfer (IET) is a fundamental component of accurately explaining the behavior of gas phase systems in which the internal energy of particular modes of molecules is greatly out of equilibrium. In this work, chemical dynamics simulations of mixed benzene/N₂ baths with one highly vibrationally excited benzene molecule (Bz^*) are compared to experimental results at 140 K. Two mixed bath models are considered. In one, the bath consists of 190 N₂ and 10 Bz, whereas in the other bath, 396 N₂ and 4 Bz are utilized. The results are compared to results from 300 K simulations and experiments, revealing that Bz^*-Bz vibration-vibration IET efficiency increased at low temperatures consistent with longer lived "chattering" collisions at lower temperatures. In the simulations, at the Bz^* excitation energy of 150 kcal/mol, the averaged energy transferred per collision, ⟨ΔE_c⟩, for Bz^*-Bz collisions is found to be ∼2.4 times larger in 140 K than in 300 K bath, whereas this value is ∼1.3 times lower for Bz^*-N₂ collisions. The overall ⟨ΔE_c⟩, for all collisions, is found to be almost two times larger at 140 K compared to the one obtained from the 300 K bath. Such an enhancement of IET efficiency at 140 K is qualitatively consistent with the experimental observation. However, the possible reasons for not attaining a quantitative agreement are discussed. These results imply that the bath temperature and molecular composition as well as the magnitude of vibrational energy of a highly vibrationally excited molecule can shift the overall timescale of rethermalization.